The present invention relates to a rotational signal detecting apparatus suitable for detecting the rotation speed or crank angle phase of an internal combustion engine.
In conventional methods for detecting the rotation speed and the crank angle phase, an electromagnetic pickup or a photoelectric pickup is used. In an example of the detection method using an electromagnetic pickup, a reluctor having a plurality of projections is fixed to the rotation shaft rotating in synchronism with the rotation of the engine to produce an alternating field varying in synchronism with the rotation of the engine, and the alternating field is detected by using a electromagnetic pickup coil.
In an example of the detection method using a photoelectric pickup, a slit plate is attached to the rotation shaft, and the light of a light emitting diode is applied to a light receiving diode via a slit of the slit plate to vary the amount of light received by the light receiving diode in synchronism with the rotation of the engine.
The rotation speed and the crank angle phase are basic information for controlling the internal combustion engine. Once the rotation speed or the crank angle phase becomes undetectable, i.e., once the apparatus for detecting these signals becomes faulty, the engine cannot be run normally. In a scheme proposed recently, at least two systems of detecting apparatus are disposed when these important signals are to be detected. Under the normal condition, one system is actually used and the other system is reserved for backup. Should one system fail, the other system is used to detect the information.
An example of such a scheme using two systems of electromagnetic pickups is described in a journal entitled "Nikkei Mechanical" published in Japan on Dec. 22, 1982, pp. 81-89. In this case, the crank angle phase is detected by one of the electromagnetic pickups. Should the detection of the crank angle phase become impossible because of a trouble incurred in the pickup coil of the above described one electromagnetic pickup, the crank angle phase is detected by the other electromagnetic pickup.
If two electromagnetic pickups are disposed and the space housing the pickups is limited, however, the two electromagnetic pickups must be disposed close together. Accordingly, the magnetic fields of the pickups interfere each other, resulting in deteriorated signal detection precision.
It is also proposed to use two or more systems of photoelectric pickups. Since dust or the like is deposited on the light emitting face of the light emitting device and the light receiving face of the light receiving device with the elapse of time, the amount of the received light is decreased, resulting in deteriorated signal detection precision. Further, a large space is demanded since two slit plates are disposed. Accordingly, it is not so desirable to dispose two systems of photoelectric pickups and use one pickup as the backup for the other pickup.